1. Field of the Disclosure
In a cellular system, in order to enhance the degree of freedom of cell configuration, it has been considered that the function of a base station is divided into a signal processing unit (BBU: Base Band Unit) and an RF unit (RRU: Remote Radio Unit) to achieve a physically separated configuration. In this case, a radio signal is transmitted between the BBU and the RRU by an RoF technique. Although the RoF technique can be broadly divided into an analog RoF technique and a digital RoF technique, recently, the digital RoF technique excellent in transmission quality has been actively studied, and use formulation has been progressed under a standards body such as CPRI (Common Public Radio Interface) (see, for example Non Patent Literature 1). As a transmission medium between the BBU and the RRU, although a coaxial cable, an optical fiber, or the like is used, particularly when the BBU and the RRU are connected through the optical fiber, a transmission distance can be dramatically extended.
The single BBU can store the plurality of RRUs, whereby the BBUs required for each of the RRUs can be aggregated into a single one, so that operation/installation cost can be reduced. As an example of such a form, as shown in FIG. 2-15, a form in which BBU 110 and RRU 120 are connected through PON (Passive Optical Network) is proposed. In this method, although a band between OLT (Optical Line Terminal) 140 and an optical splitter is fixed, a band between the optical splitter and ONU (Optical Network Unit) 150 can be changed according to a required band of the ONU 150. As a signal multiplexing method of the PON, TDM, WDM, FDM, or the like can be employed. The application area of the present disclosure is not limited to FIG. 2-15, and the disclosure is applicable to a case in which the BBU 110 stores one or more RRUs 120.
The present disclosure relates to a required band reduction technique between the BBU 110 and the RRU 120.
2. Discussion of the Background Art
Hereinafter, a digital RoF transmission technique between the BBU 110 and the RRU 120 is referred to as a related technique. A link which converts a digital signal (IQ data) for each I axis and Q axis of a radio signal generated by the BBU 110 into an optical signal, transmits the optical signal to the RRU 120, converts the optical signal received by the RRU 120 into a radio signal, and transmits the radio signal to a terminal is referred to as a downlink. Meanwhile, a link which receives a radio modulation signal transmitted from a terminal in the RRU 120, converts the received radio signal into an optical signal, transmits the optical signal to the BBU 110, converts the optical signal received by the BBU 110 into IQ data, and demodulates a signal is referred to as an uplink.
A device configuration example of RRU related to the present disclosure is shown in FIG. 2-16.
For uplink signal processing, the RRU 120 has an antenna 11 which transmits/receives a radio signal, a transmission/reception switching section 12 which switches transmission/reception, an amplifier 21 which amplifies a signal power of a received radio signal to a level capable of signal processing, a down-conversion section 22 which down-converts a radio signal, an A/D conversion section 23 which converts a down-converted analog signal into IQ data, a base band filter section (uplink) 24 which applies filtering processing to the IQ data, a frame conversion section 25 which multiplexes the IQ data and a control signal, and an E/O conversion section 26 which converts an electrical signal into an optical signal and transmits the optical signal. The transmission/reception switching section 12 can correspond to both FDD (Frequency Division Duplex) and TDD (Time Division Duplex).
Meanwhile, for downlink signal processing, the RRU 120 has an O/E conversion section 31 which converts an optical signal received from the BBU 110 into an electrical signal, a frame conversion section 32 which takes out a control signal and IQ data from a received signal, a base band filter section (downlink) 33 which applies filtering processing to the IQ data, a D/A conversion section 34 which converts the IQ data into an analog signal, an up-conversion section 35 which up-converts an analog signal, an amplifier 36 which amplifies electric power to a given transmitted power, the transmission/reception switching section 12, and the antenna 11.
A device configuration example of the BBU related to the present disclosure is shown in FIG. 2-17.
For the uplink signal processing, the BBU 110 has an O/E conversion section 41 which converts an optical signal into an electrical signal, a frame conversion section 42 which takes out a control signal and IQ data from a received signal, and a modulation/demodulation section 43 which demodulates the IQ data.
Meanwhile, for the downlink signal processing, the BBU 110 has the modulation/demodulation section 43 which outputs the IQ data of a radio modulation signal, a frame conversion section 51 which multiplexes the IQ data and a control signal, an E/O conversion section 52 which converts an electrical signal into an optical signal and transmits the optical signal, and a control signal generation section 50 which generates the control single using a signal for synchronization and so on.
In cellular systems such as LTE (Long Term Evolution) and WiMAX (Worldwide Interoperability for Microwave Access), in order for a terminal to transmit and receive user data, a communication channel (radio band) specific to a terminal is required. The radio band is allocated by a base station. Taking an LTE system as an example, as shown in FIG. 2-18, a base station performs scheduling with a period of 1 ms at minimum and allocates the radio band to each terminal. In FIG. 2-18, white portions show unused resource blocks, and hatched portions show allocated resource blocks.
The allocation of the radio band is performed in the unit of resource block (RB), and 1 RB is 180 kHz and 0.5 ms. When a system band width is 20 MHz, 110 RBs exist on a frequency axis. In 1 RB, when a normal cyclic prefix is supposed, seven symbols (one symbol is 71.4 μs including the cyclic prefix) are inserted.
In the related technique, a sampling frequency fs used in the conversion of a radio modulation signal into a digital signal is determined by a system band width. Taking CPRI as an example, when the system band width of LTE is 20 MHz, fs=30.72 MHz, and when the system band width is 10 MHz, fs=15.36 MHz. As shown in FIG. 2-19, a sampling cycle Δs which is a time interval between signals obtained by quantizing radio signals is constant, and the sampling frequency fs is not changed according to time.
Taking a case, where an LTE (Long Term Evolution) signal is transmitted through CPRI, as an example, the sampling frequency of 30.72 MHz is used for a system having a system band width of 20 MHz. In digital sampling for each I axis and Q axis, a quantization bit number of 4 to 20 bits is applied to an uplink signal, and a quantization bit number of 8 to 20 bits is applied to a downlink signal. In the frame conversion section, a control signal is inserted into 1/16 of the entire frame, and the signal is transmitted after 8 B/10 B encoding.
Meanwhile, in the cellular systems such as LTE and WiMAX (Worldwide Interoperability for Microwave Access), in order for a terminal to transmit and receive user data, a communication channel (radio band) specific to a terminal is required. The radio band is allocated by a base station.
Taking the LTE system as an example, as shown in FIG. 2-18, the base station performs scheduling with a period of 1 ms at minimum and allocates the radio band to each terminal. The radio band allocation is performed in the unit of resource block (RB), and 1 RB is constituted of a frequency domain of 180 kHz and a time domain of 0.5 ms. When the system band width is 20 MHz, 110 RBs exist on the frequency axis. In 1 RB, when the normal cyclic prefix is supposed, seven symbols (one symbol is 71.4 μs, including the cyclic prefix) are inserted.